This invention relates in general to measurement of samples and in particular, to a system for a non-destructive measurement of properties of a sample. This invention is particularly useful for non-destructive measurement of dose of a dopant in a semiconductor material.
In semiconductor manufacturing, it is frequently desirable to obtain information concerning the properties of a semiconductor material in a sample such as a wafer during or after processing. For example, it may be desirable to obtain an indication of the dose of a dopant in a semiconductor material such as silicon.
One system for measuring dose is proposed in U.S. Pat. No. 4,952,063. As described in this patent, a thermal wave is generated in a material by causing periodic localized heating of the material by focusing an intensity modulated pump beam of light on a spot of the sample surface. A probe beam is directed towards the spot of the sample surface to sense changes in the indices of refraction induced by the pump beam. The output of the detector is processed to analyze reflected light signals that are in phase with the modulation of the pump beam in order to detect changes in reflectivity of the sample. Such reflectivity measurements are then used to determine dopant concentration, residue deposits and defects of samples.
While the system described in U.S. Pat. No. 4,952,063 may be useful for certain dose measurements, it has many disadvantages. Thus, for some dopant concentrations, the change in reflectivity measured may correspond to different values of the dopant concentration so that the dopant concentration cannot be uniquely identified from the change in reflectivity. The change in reflectivity may also be too small to be measured for some dopant concentrations.
Spectroscopic ellipsometry is used to measure the damage profile at high doses. For wafers implanted at low doses, spectroscopic ellipsometry may not be adequate.
In U.S. Pat. No. 5,536,936, a spectroscopic ellipsometer is used to measure the ellipsometric parameter values of the sample in the presence of and in the absence of excitation of the sample. The technique in U.S. Pat. No. 5,536,936, however, lacks adequate sensitivity for measuring the change in ellipsometric parameters caused by excitation of the sample.
None of the above described systems is entirely satisfactory. It is, therefore, desirable to provide an improved system for non-destructive measurement of samples, which avoids the above-described difficulties or disadvantages.